Nitride semiconductors have been recently produced as materials used for a high bright blue LED and a pure green LED, a full color LED display and a traffic signal LED. Such LEDs are provided with an active layer of SQW (Single Quantum Well) or MQW (Multi Quantum Well) where the well layer is made of InGaN and positioned between a p-type nitride layer and an n-type nitride layer to form a DH (Double Hetero) structure. The wavelength of the blue or green light emitting from the active layer depends on a ratio of In in the InGaN well layer.
The inventors have first realized laser emitting by using the above nitride materials and reported it in Jpn. J. Appl. Phys. 35(1996) L74 and Jpn. J. Appl. Phys. 35(1996)L217. The laser device comprises the DH structure where the active layer is MQW having InGaN well layers and showed the following data:
Threshold current: 610 mA; PA1 Threshold current density: 8.7 kA/m2; PA1 Wavelength: 410 nm PA1 Threshold current: 187 mA; PA1 Threshold current density: 3 kA/m2; PA1 Wavelength: 410 nm
(pulse width 2 .mu.m and pulse cycle 2 ms)
The inventors have further improved the laser device and reported it in Appl. Phys. Lett. 69(1996)1477. The laser device comprises a ridge strip structure formed on a part of p-type nitride semiconductor and showed the following data.
(pulse width 2 .mu.m, pulse cycle 2 ms and duty ratio: 0.1%)
The inventors have first succeeded in CW (Continuous-Wave) Oscillation or Operation at room temperature and reported it in Gijutsu-Sokuho of Nikkei Electronics issued on Dec. 2, 1996, Appl. Phys. Lett. 69(1996) and Appl. Phys. Lett. 69(1996)4056.
The laser diode showed a lifetime of 27 hours at 20.degree. C. under the threshold current density of 3.6 ka/cm.sup.2, the threshold voltage of 5.5 V and the output of 1.5 mW.
On the other hand, the blue and green LED of nitrides showed a forward current (If) of 20 mA and a forward voltage (Vf) of 3.4 to 3.6 V which are higher by 2 V or more than those of red LEDs made of GaAlAs semiconductors. Therefore, further decrease of Vf in the blue and green LED was required. Additionally, there was required an effective LD which can decrease the threshold current and voltage to get a longer lifetime of CW operation at room temperature, because the conventional LD still had a higher threshold current and voltage.
The inventors have gotten the idea that technology of decreasing the threshold in LDs was applicable to LEDs in order to decrease the Vf. Therefore, a first object of the present invention is to decrease the threshold current and voltage of nitride semiconductor LDs and realize a longer lifetime of CW operation.
In the specification, it should be understood that the general formulae: In.sub.x Ga.sub.1-x N and Al.sub.y Ga.sub.1-y N show chemical atoms which compose of nitride layers and therefore, even if different layers are represented by the same formula, the different layers do not necessarily have the same composition, that is, the same x or y does not mean the same ratio.